This invention relates to a method and a device for matching fingerprints.
The word "fingerprint" is herein used as a representative of a fingerprint or a like pattern or figure. More particularly, the finerprint may be an actual finger, a palm print, a toe print, a soleprint, a squamous pattern, and a streaked pattern composed of streaks. The fingerprint may also be a diagram drawn by a skilled person to represent a faint fingerprint remain which is, for example, left at the scene of a crime.
The "matching" is for recognition of a fingerprint with reference to a plurality of known fingerprints. The matching may also be for discrimination, collation, and/or identification of the fingerprint.
A fingerprint to be matched with a known fingerprint, is usually called a search fingerprint. The known fingerprint is called a file fingerprint. Each fingerprint is featured by ridges and more specifically by minutiae.
It is conventional on carrying out the fingerprint matching to select a coordinate system on each fingerprint as will later be described with reference to a few of about forty figures of the accompanying drawing. The coordinate system is used to represent the position or location which each minutia has on the fingerprint. Furthermore, a direction is defined together with the sense in connection with each minutia with reference to the coordinate system. In addition to the position and direction data of each minutia, a local feature characteristic to each minutia is known which is independent of the coordinate system. An example of such a local feature is the difference between types of minutiae. Other examples are "relation data" which are representative of relationships between each minutia and adjacent minutiae. The relation data are substantially independent of the coordinate system as will become clear as the description proceeds with reference to the accompanying drawing. At any rate, the coordinate system will be called a principal coordinate system.
Fingerprint matching has been carried out by comparing the minutia position and direction data and relation data of a search fingerprint with those of each file fingerprint. Inasmuch as the coordinate systems are independently selected on the respective fingerprints, the position and direction data of a fingerprint must be transformed so as to be represented by a coordinate system which best matches with the coordinate system selected on the other fingerprint.
The best match has been attained only with time consuming procedures. Even with the best match, the fingerprint matching has been unreliable due to various circumstances under which the fingerprints are printed. This has resulted in unsatisfactory performance of a conventional method of fingerprint matching and of a conventional fingerprint matching device. Incidentally, the position and direction data given either by the principal coordinate systems or by a coordinate system into which at least one of the coordinate systems is transformed to provide a certain degree of match, will be called original position and direction data in contrast to transformed position and direction data which are obtained after specific coordinate transformation.